The present invention relates to a superconducting-coil apparatus in which a superconducting coil unit is cooled by superfluid helium, and more specifically, to a superconducting-coil apparatus in which current lead wires, connected between an exciting current source and a superconducting coil unit, are cooled to avoid burning, in the event of quenching.
In superconducting-coil apparatuses of this type, when liquid helium is cooled to a normal-fluid state, a superconducting coil is changed from normal-conducting state to superconducting state. When liquid helium is further cooled to change from normal-fluid state to superfluid state (.lambda.=2.17 K), the superconducting coil is changed from superconducting state to highly stable superconducting state. If the coils are excited in the superconducting state, they produce a high-intensity magnetic field, without any substantial electrical loss.
One such prior art superconducting-coil apparatus is disclosed in U.S. Pat. No. 3,992,893. This apparatus is provided with a cryostat, which includes a superfluid helium bath and a normal-fluid helium bath. A liquid helium contained in the superfluid helium bath is maintained in the superfluid state and a liquid helium contained in the normal-fluid helium bath is maintained in the normal-fluid state. The normal-fluid bath and superfluid bath are thermally insulated by an insulator and coupled to each other through a channel and a superconducting coil unit is immersed in the superfluid helium.
In a conventional superconducting-coil apparatus disclosed in a document, lead wires for supplying a current to the superconducting coil unit are extended from the exciting current source to the superconducting coil through the normal fluid bath, the channel and the superfluid bath so that the lead wires are maintained at a sufficiently low temperature. Furthermore, an insulating member is detachably fitted in the channel to prevent heat from being transferred between the baths through the channel.
The superconducting coil unit may sometimes undergo quenching while it is being excited. The quenching is a phenomenon where the coil unit changes from superconducting state to normal-conducting state. If such quenching occurs, a large amount of electrical energy, stored in the coil unit, might possibly break the coil unit. In the event of quenching, therefore, the current supply to the coil unit is cut off, so that the excitation of the coil unit is interrupted. At the same time, the current lead wires extending from the coil unit are shorted by an electric resistor, which is connected between the lead wires. As a result, the electrical energy in the coil unit is dissipated.
In the normal-conducting state, however, the superconducting coil unit has an electric resistance. Therefore, part of the electrical energy is converted into Joule heat, in the coil unit, so that the coil unit is heated. The Joule heat, delivered from the heated coils, is transmitted through the current lead wires. When the heat reaches those portions of the lead wires inside the channel, it is stored in the channel, in which the insulator is inserted. As a result, these wire portions may possibly be burned out, without being cooled substantially.